The present invention concerns a seal for sealing a gap between relatively rotatable components.
Seal assemblies are widely applied to seal a gap between relatively rotatable components; for example, the gap between a revolving shaft or axle and the housing bore of a motor. The seals serve to retain lubricant within the motor and to exclude the entry of contaminants. Both of these aspects are important for proper running of the motor and the prevention of early failure. A typical seal comprises an outer casing to which a sealing element is bonded. Elastomeric materials, such as nitrile rubber, may be used for the sealing element. The outer casing seals statically against the housing bore. The sealing element seals against a counterface of the e.g. shaft and provides dynamic sealing during rotational conditions and static sealing during stationary conditions.
Seals that are designed to retain oil or a low-viscosity fluid are generally spring loaded, to ensure static and dynamic tightness. In other words, the sealing element has a certain preload to keep it pressed against the rotating component and this leads to additional friction and heat generation at the seal interface.
Another factor which must be taken into account with regard to seals is misalignment. This can be shaft-to-bore misalignment, which is the amount by which the shaft is off centre with respect to the centre of the housing bore; dynamic runout, which is a measure of the amount by which the shaft does not rotate around its true centre; or a combination of both. Due to manufacturing and assembly inaccuracies, some misalignment and runout is always present and seals are generally designed to accommodate these eccentricities up to predefined maximum values. In applications where significant misalignments must be accommodated, a high spring preload in a lip seal is advantageous. In a crankshaft sealing application, for example, random shaft defections caused by piston slap may occur, and a high preload helps ensure that the sealing element remains in contact with the shaft, despite the rotational eccentricities. The higher preload, however, increases the friction, which leads to a higher energy consumption and greater energy losses.